Recently, in the field of planographic printing, a computer-to-plate technique has been developed in which a planographic printing plate precursor is directly subjected to laser exposure, without via a lithographic film, based on digital data from a computer or the like to produce a plate, and a high-sensitivity laser recording type planographic printing plate for use in such a technique has been developed.
However, the conventional high-sensitivity laser recording type planographic printing plate has a problem such that, when it is subjected to exposure using an Ar (488, 514.5 nm) or FD-YAG (532 nm) laser internal drum type platesetter, which is most commonly used on the market, a fog is likely to occur. For example, it is assumed that a negative plate is used as a photosensitive material, and a solid fill image is projected onto the entirety of one side of the plate by exposure. When the other side is a non-image portion, a thin fog (like when development is insufficient) occurs. When the other side is a halftone dot (about 140 to 220° where the other side is at 180° with respect to a light source), a defect, such a thickened dot or the like, occurs. Therefore, there is a demand for an improvement in the conventional technique.
A conventional high sensitivity printing plate for use in plate production using laser, such as Ar laser or FD-YAG laser, may be taken out of a cardboard package under red light, may be loaded into a platesetter cassette, or may be manually inserted into a platesetter. All of these pieces of work need to be performed under dim red light, resulting in considerably poor workability. Compared to this, a typical diazo printing plate can be handled under yellow light or UV-filtered white light, resulting in better workability. Thus, there has been a large demand for an improvement in safe light property of the high sensitivity laser recording planographic printing plate on the market in terms of workability.
To meet these demands, Patent Document 1 describes a method of producing a planographic printing plate by subjecting a planographic printing plate precursor having (A) an aluminum support and (B) a laser photosensitive recording layer to exposure using an internal drum type platesetter which employs semiconductor laser light in the range of ultraviolet to a visible region (360 to 450 nm). According to this plate producing method, it is described that a plate can be handled under yellow light, and a fog does not occur even when exposure is performed using an internal-drum platesetter.
On the other hand, in conventional planographic printing plate precursor producing processes, a step of dissolving and removing an unnecessary portion of an image recording layer using a developer solution or the like after exposure, is required. One of the recent challenges is to eliminate or simplify such a wet process which is additionally performed.
As a simple plate producing method which meets this challenge, for example, a method called “development-on-machine” has been proposed in which a planographic printing plate precursor which has an image recording layer capable of being dissolved or dispersed in printing ink and/or fountain solution is used, and printing ink and/or fountain solution are supplied to an exposed plate attached on a printing machine to remove an unexposed portion of the image recording layer, thereby obtaining a planographic printing plate.
Examples of a specific development-on-machine method include: a method of using a planographic printing plate precursor having an image recording layer capable of being dissolved or dispersed in a fountain solution, an ink solvent, or an emulsion of a fountain solution and an ink; a method of physically removing an image recording layer by means of contact with rollers or a blanket of a printing machine; a method of physically removing an image recording layer by means of contact with rollers or a blanket after weakening the aggregation force of the image recording layer or the adhesion force of the image recording layer and a support by means of permeation of a fountain solution, an ink solvent, or the like; and the like.
Note that, as used herein, the term “development step” refers to a step of removing an image recording layer of a planographic printing plate precursor by means of contact with liquid (typically, an alkaline developer solution) using an apparatus (typically, an automatic developing machine) other than a printing machine, thereby exposing a hydrophilic support surface, and the term “development-on-machine” refers to a method and a step of removing an image recording layer of a planographic printing plate precursor by means of contact with liquid (typically, printing ink and/or fountain solution) using a printing machine, thereby uncovering a hydrophilic support surface, unless otherwise specified.
Thus, recently, there is an increasing demand for streamlining of plate producing work in terms of both digitalization and simplification, dry type, and non-process of plate producing work.
To meet this demand, for example, Patent Document 2 describes that a photosensitive composition containing polyvinyl pyrrolidone, polycarboxylic acid polymer, an olefinically unsaturated monomer, a photoinitiator, and the like is used to obtain a printing plate capable of being attached to a printing machine immediately after exposure and without an additional process. However, the photoinitiator used therein has a problem on sensitivity and a fog after exposure.
Patent Document 3 describes a photosensitive planographic printing plate (planographic printing plate precursor) which has a photosensitive layer (image recording layer) having a double-layer structure composed of a photosensitive hydrophilic layer and a photosensitive hydrophobic layer thereon. This photosensitive planographic printing plate can be attached to a printing machine immediately after exposure and without an additional process. However, the adhesion between the upper layer and the lower layer is insufficient, so that printing durability is still poor.
Patent Document 4 describes a planographic printing plate precursor in which an image formation layer obtained by dispersing a hydrophobic thermoplastic polymer particle in a hydrophilic binder is provided on a hydrophilic support. Patent Document 4 also describes that, after exposed with infrared laser, the planographic printing plate precursor can be thermally coalesced with the hydrophobic thermoplastic polymer particle so that an image can be formed, and thereafter, the plate can be attached to a cylinder of a printing machine, followed by development-on-machine using fountain solution and/or ink.
However, it was found that, in the case of the above-described method of forming an image by coalescence of the microparticle due to simple thermal fusion bonding, although good development-on-machine ability is exhibited, image strength is weak, so that printing durability is insufficient.
Therefore, it has been proposed that a reaction in an image recording layer is utilized to improve printing durability. For example, Patent Document 5 describes a negative-type planographic printing plate precursor which employs a photosensitive composition containing a polymer having an acid group or a salt thereof (functional group) and a photocurable functional group, and a photoinitiator, and is usable in printing after exposure without via a development step. Patent Document 6 describes a planographic printing plate precursor which employs a photosensitive composition containing, on a surface-treated aluminum support, (a) a water-soluble or water-dispersible polymer, (b) a monomer or an oligomer having at least one photopolymerizable ethylenically unsaturated double bond, and (c) a specific photopolymerization initiating system having a λmax of 330 nm or more and 375 nm or less, and is usable in printing after exposure without via a development step.
On the other hand, in the field of a photopolymerizable printing plate, it is known that a middle layer containing a compound having a polymerization reactive group and a support adsorptive group is provided between a support and a photopolymerization layer in order to improve the adhesion therebetween.
For example, Patent Document 7 discloses a sol-gel middle layer provided between a support and a photopolymerization layer. Patent Document 8 discloses a sol-gel middle layer to which a phenolic compound or a phosphate compound is added. In these middle layers, a developer solution containing silicate is exclusively used in order to cause silicate to be bound to a non-image portion during development to improve hydrophilicity.
Patent Document 9 discloses a middle layer made of an organic phosphate compound, in which a phosphono group, which is an acidic group, is quickly dissociated during an alkali development process, and loses interaction with a support, and therefore, which is at least partially removed from a non-image portion, or is conferred a high level of hydrophilicity at a surface due to dissociation.
However, all techniques for attempting to achieve both the adhesion between a support and a photopolymerization layer and an improvement in resistance to stain using these adhesive middle layer, are provided based on the assumption of an alkali development action after exposure. When these techniques are applied to a development-on-machine-type planographic printing plate precursor, an improvement in the adhesiveness leads to a degradation in resistance to stain.
On the other hand, when an image recording method which utilizes light of ultraviolet to a visible region is applied to such a non-processed printing plate which does not require a development process using a developer solution, an image recording layer is not fixed and is still kept photosensitive to room light even after exposure. Therefore, after a planographic printing plate precursor is taken out of its package, the plate needs to be handled in a completely light-tight situation or under the environment of safe light until development-on-machine is completed. If otherwise, i.e., for example, if the plate is handled under white light after exposure, a fog occurs in an image recording layer, the layer remains in a non-image portion, leading to printing stain. Therefore, even printing machine work which conventionally does not require safe light needs to be performed under the environment of safe light, resulting in interference with printing work, such as color adjustment or the like. Therefore, there is a demand for a non-processed printing plate system capable of being performed under white light.
[Patent Document 1] JP-A No. 2000-35673
[Patent Document 2] JP-A No. 47-8657
[Patent Document 3] JP-A No. 6-502931
[Patent Document 4] JP-B No. 2938397
[Patent Document 5] International Publication WO96/34316
[Patent Document 6] JP-A No. 2000-39711
[Patent Document 7] JP-A No. 7-159983
[Patent Document 8] JP-A No. 9-269593
[Patent Document 9] JP-A No. 2000-235254